1. Field of the Invention
The present invention relates to a semiconductor laser whose oscillation polarization mode can be changed by an external control and which is suitable for use as a light source apparatus in the field of optical communications, for example. The present invention further relates to an optical communication system and a light transmitter which use the above-mentioned semiconductor laser, respectively.
2. Related Background Art
As a conventional device, Japanese Patent Application Laid-Open No. 2(Heisei)-159781 discloses a distributed feedback (DFB) laser whose oscillation polarization mode can be changed by an external control. In this DFB laser, carrier injection for phase adjustment and formation of population inversion causes a change in the phase of internal light, and the oscillation occurs in either of transverse electric (TE) and transverse magnetic (TM) modes whose threshold gain is lower.
In the above-discussed device, however, the oscillation polarization mode of the device output is changed by controlling the phases of light in TE and TM modes. Therefore, a predetermined amount of current always needs to be injected into the device to form population inversion. Further, in the prior art device, an active layer is formed with a bulk material, and hence a difference in gain between TE and TM modes is small. However, if an active layer having a quantum well structure is used, the difference in gain between TE and TM modes becomes large. As a result, a wavelength range, in which the polarization mode of oscillated light can be changed by the same operation principle as the bulk material case, is narrowed, and the fabrication precision needed for formation of a grating and the like is strict. Here, the pitch of the grating determines Bragg wavelengths for TE and TM modes which are respectively oscillation wavelengths of light in TE and TM modes.
Furthermore, the prior art device is not provided with an effective and positive means for enabing the oscillation polarization mode switching. Therefore, it is difficult to obtain a highly reproducible polarization mode selective laser.